1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition, and particularly to a lead-free piezoelectric ceramic composition with a suitable amount of CeO2 or CeO2-containing complex additives. Also, the present invention relates to a method for preparing the same.
2. Background Art
In the past, as a piezoelectric ceramic composition, a Pb(Zr,Ti)O3 (PZT) component-based ceramic containing lead has been used as the most important piezoelectric compound. This is because the above PZT exhibits large piezoelectric properties and has a high mechanical quality factor superior in long-term stability and enables the easy fabrication of materials of various properties required for applications such as sensors, actuators, and filters. Further, PZT has a high relative dielectric constant, so can also be used as a capacitor, etc.
While a piezoelectric ceramic composition comprised of PZT has superior properties, it includes lead among its component elements, so harmful lead such as volatile and harmful lead oxide leaches out from the industrial waste of products containing the PZT, and therefore manufacture and disposal of devices using it will cause severe environmental pollution problem. The rising awareness of such environmental issues in recent years has made difficult the production of products that might become causes of environmental pollution such as with PZT. Therefore, development of a piezoelectric ceramic composition containing no lead is desirable.
In general, lead-free piezoelectric ceramics can be mainly divided into three types in terms of their structures: perovskite, tungsten bronze and bismuth layer. The lead-free piezoelectric ceramics in tungsten or bismuth layer structure are featured with high Curie temperature and are especially suitable for high temperature applications. In perovskite type lead-free piezoelectric ceramics, (Bi1/2Na1/2) TiO3-based ceramics (BNT) with rhombohedral perovskite structure are considered to be good candidates to replace PZT ceramics because of its strong ferroelectricity. However, as comparing with PZT ceramics, BNT ceramics have low piezoelectric properties and low depolarization temperature. In contrast, (Na,K)NbO3 (NKN)-based ceramics exhibit high Curie temperatures, large electromechanical properties and low densities, showing attractive potential for replacing PZT ceramics in some practical use.
Although (K,Na)NbO3-based ceramics have comparatively excellent piezoelectric properties, they are hard to sinter. As a result, it is featured with high dielectric loss, narrow sintering temperature range and poor reproducibility. Practically, a hot pressing technique is required to form a highly dense structure of (K,Na)NbO3 so as to give excellent piezoelectric properties, which results in high cost and low efficiency in manufacture of the desired devices. The poor sinterability of (K, Na)NbO3 is due to the volatility of alkali metal components (K, Na) and a low melting point of KnbO3, which tend to cause abnormal growth of grains and make it difficult to increase the sintering temperature to a level high enough to assure high densification of the structure.
As a result, studies are made to improve the sinterability of (K,Na)NbO3-based ceramics. For example, in JP2004-244301 and JP2004-244302, it is described that the substitution of Nb by Sb and Sb+Ta in the form of a solid solution improves its sinterability. However, these compositions show narrow sintering temperature range and poor reproducibility, which is crucial for practical use. Furthermore, their piezoelectric and dielectric properties need to be improved.